Watermarking is an effective tool for tracing the source and distribution path of content items, such as movies, pictures, songs, radio and television, which could be made available in streams or downloadable form. One application of watermarking is distribution monitoring where distributed, broadcast material is constantly monitored for the presence of watermarks. When a watermark is found in a segment, the watermark or an identifier embedded therein links the segment to a source and a program. This enables an agency which performs the task of monitoring distribution of content to verify the distribution of content items generated by various agencies involved in the process of distribution and check whether each agency has fulfilled its obligations in the process of distribution.
A distribution monitoring agency may operate the watermark detectors to generate reports for its customers who have inserted their unique identifiers using watermarking. While such a watermark detection process can be done on multiple locations to cover local stations, monitoring report generation is often done from a central place. It is also possible that some information that links the segments' meta-data to unique identifiers used for watermarking is send by the customers to the service provider to be included in their reports. (These monitoring reports also incorporate meta-data from other sources such as programming information from EPG.)
Managing the various watermarks can be a challenging task. In a typical distribution chain, multiple parties are involved with the generation, editing (mixing) and distribution and broadcast of the content. Each of these parties may want to insert its own watermarks, to allow monitoring of the distribution of its content. As a result, content segments may comprise watermarks from multiple parties.
For example, consider a scenario where a news agency A makes a recording of a news event. Unedited footage is distributed to many news agencies. A news agency B1 takes one or more segments out of agency A's recording and edits it into a composite content item together with segments from a news story that agency B1 itself prepared. This composite content item, the edited news story is then distributed to many broadcast networks. Similarly, another news agency B2 takes the same segment from A's recording and creates its own news story. Finally, a television broadcaster C takes the news story from B1 and/or B2 and broadcasts it in its program flow. Agency A may want to monitor whether agencies B1 and/or B2 have distributed content obtained from A. At the same time agencies B1 and B2 may want to whether a broadcasting agency C has distributed the content from B1 and B2 respectively by broadcasting it. An agency D hired by B1 and/or B2 may be involved in monitoring the content broadcast by C and verify whether B1's content was correctly distributed, i.e. verify whether all content item segments have been distributed or broadcast
To keep track of the relevant segments from the various parties, i.e. monitor broadcast content for segments from the news agencies A, B1, B2, it is known to have each agency insert its own watermarks in each segment, that can be coupled to the agency in question. For example, agency A may insert a watermark with a payload that contains A's name in every segment. Agencies B1's and B2's news stories would then contain a segment with that payload, allowing agency A to track the use of that segment (and in turn to for example invoice B1 and B2 for the use). Similarly, agencies B1 and B2 would insert their own watermarks in their news stories, for example an identifier of the story and the station, or an identifier and a timestamp.
The broadcast content can be monitored by receiving the broadcast content and detecting watermarks in the segments constituting the content. The watermark inserted by agencies B1 and/or B2 would then be detected when broadcaster C broadcasts that news story and the broadcast news story is received and monitored. Broadcaster C may in turn itself watermark its broadcasts to e.g. allow detection of unauthorized retransmissions by its subscribers.
A consequence of this approach is that certain segments of the content contain multiple watermarks. The segments taken from agency A's content by agencies B1 and B2 that end up in C's broadcast may contain three watermarks: A's original identification, B1 and/or B2's story and station identification and C's own identification to detect retransmissions.
It is well-known that adding multiple watermarks to a segment of content is undesirable, as this often leads to visible or audible distortions or a conflict where the addition of a new watermark corrupts a previously embedded watermark. Monitoring of broadcasts thus becomes unreliable, since corrupted watermarks can no longer be detected.
One may of course control the embedding process by reducing watermark strength to ensure that multiple watermarks can be accommodated, but then each watermark will be of significantly reduced strength compared to what would be possible. In this case, the robustness (detectability) of the watermarks will degrade as there is less watermark signal to be detected. In addition, this increases the chances that an attacker may remove one or more of the watermarks. Moreover, multiple watermarks often impede the detection of each other due to interference.
Therefore, multiple watermarks are not used in many practical systems. Instead, each party or agency embedding watermarks can detect the presence of another watermark in a content segment and temporarily pause the embedding of the watermark. This however has the disadvantage that the monitoring of broadcasts becomes unreliable due to pauses in the embedding of watermarks.